The present invention relates to an Fe group-based amorphous alloy ribbon which has magnetic characteristics exhibiting a large Barkhausen discontinuity in a magnetic hysteresis loop and which has excellent pulse voltage generating properties. More particularly, the present invention relates to a magnetic marker comprising the above ribbon for use in an anti-theft system or in an article surveillance system.
It is well known that amorphous alloy materials having various forms such as a ribbon form, a filament form, a powder form, etc., can be obtained by quenching a molten alloy. In particular, the Fe- and Co-based amorphous alloy filaments disclosed in JP-A-1-25941 (corresponding to U.S. Pat. No. 4,735,864) and JP-A-1-25932 (corresponding to U.S. Pat. No. 4,781,771) are known magnetic materials having a distinctive magnetic characteristic called a large Barkhausen discontinuity. These materials undergo a sudden magnetic flux reversal when the strength of an applied magnetic field reaches a critical value in a magnetic hysteresis loop. (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d.) These amorphous alloy filaments have been widely used in various magnetic markers and in magnetic sensors as pulse generator which induce a sharp voltage pulse in a detection coil independent of the alternating frequency of an applied magnetizing magnetic field.
On the other hand, it is known that a quench-solidified Fe group-based amorphous alloy ribbon does not exhibit a large Barkhausen discontinuity, while a quench-solidified amorphous filaments exhibit a large Barkhausen discontinuity. However, it is also known that an amorphous alloy ribbon subjected to a specific heat treatment is capable of exhibiting a large Barkhausen discontinuity. JP-B-3-27958 (corresponding to U.S. Pat. Nos. 4,660,025 and 4,686,516) discloses that, by keeping an Fe-based amorphous alloy ribbon in a flattened state after heat treating at 380xc2x0 C. with twist of 4 turns per 10 cm length of the ribbon, the amorphous alloy ribbon exhibits magnetic characteristics having a large Barkhausen discontinuity. (The term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined published Japanese patent applicationxe2x80x9d.)
Also, EP-A-762354 discloses a Co-based amorphous alloy ribbon heat-treated by passing an electric current therethrough in a magnetic field which has magnetic characteristics exhibiting a large Barkhausen discontinuity, and also describes that magnetic markers can be formed from such a Co-based amorphous alloy ribbon.
Recently, with the popularity of anti-theft systems and article surveillance systems utilizing magnetic markers, a magnetic marker having an inconspicuous construction for adhering to articles has been desired, and there is a demand for a new small-sized soft magnetic material having a length of 10 cm or shorter, and desirably 7 cm or shorter, which can be formed into a thin-type magnetic marker.
However, in the case of magnetic markers formed from the above-described Fe- and Co-based amorphous alloy filaments, the diameter of the filament is necessarily 90 xcexcm or larger in order to provide sufficient pulse generating characteristics. Thus, the resulting magnetic markers disadvantageously become thick when these filaments are inserted between various film materials or papers.
On the other hand, when the present inventors prepared an Fe-based amorphous alloy ribbon which was twisted 4 turns per 10 cm while being heat treated at 380xc2x0 C. for 25 minutes using an Fe81Si4B14C1 (the numerals represent atomic %) amorphous alloy ribbon having a width of 2 mm and a thickness of 25 xcexcm as disclosed in JP-B-3-27958, the following problem was identified.
That is, the present inventors found that amorphous alloy ribbons longer than 10 cm can be obtained which have magnetic characteristics exhibiting a large Barkhausen discontinuity, but a twisting number of 4 or more turns per 10 cm of the length of the ribbon during heat treatment is required. In addition, in a state in which the twisted amorphous alloy ribbon is released and held flat after heat treatment, the minimum magnitude of the applied magnetizing field (critical magnetic field) needed to evoke a large Barkhausen discontinuity is greater than 0.8 Oe. Also, because the critical magnetic field is large, an induced pulse is not generated in a detection coil in a magnetizing field of 0.7 Oe or lower. Thus, only magnetic markers having poor detection characteristics in various anti-theft systems can be realized.
Also, it has been found that an amorphous alloy ribbon having a length of 10 cm or shorter after heat treatment does not have magnetic characteristics exhibiting a large Barkhausen discontinuity. That is, it has been determined that an amorphous alloy ribbon after heat treatment where the twisted amorphous alloy ribbon is untwisted and the ribbon is held flat has poor pulse voltage generation characteristics, and thus cannot be formed into small-sized and thin magnetic markers.
Furthermore, because the twisting number is as high as 4 turns or more per 10 cm length of the amorphous alloy ribbon, there are problems in that the ribbon frequently tears during heat treatment, and kinking or distortion of the ribbon due to the severe twisting occurs when winding the ribbon on a bobbin after heat treatment or when unwinding the ribbon from a bobbin. Also, it was determined that magnetic markers comprising an Fe-based amorphous alloy ribbon which, after heat treatment is flattened with a film of an organic material, are problematic in that, due to the high toughness of the Fe-based amorphous alloy ribbon, the magnetic markers adopt a strongly twisted state. Handling of the magnetic marker thus becomes difficult, and the magnetic markers are liable to release from articles to which they are adhered.
Also, the present inventors heat treated a Co-based amorphous alloy ribbon by passing electric current therethrough in a magnetic field as disclosed in EP-A-762354. The magnetic characteristics thereof were measured. It was determined that an amorphous alloy ribbon having a length of 10 cm can exhibit a large Barkhausen discontinuity, but the minimum value of the magnetizing field (critical magnetic field) needed to evoke a large Barkhausen discontinuity is larger than 0.8 Oe. Also, it was confirmed that, because the critical magnetic field for the amorphous alloy ribbon is large, magnetic markers formed with this amorphous alloy ribbon do not generate an induced pulse in a detection coil in a low magnetizing field of 0.7 Oe or lower. Thus, the detection characteristics in various anti-theft systems are poor, and practical magnetic markers cannot be obtained.
Accordingly, the development of an amorphous alloy material which has magnetic characteristics exhibiting a large Barkhausen discontinuity even in a length of 10 cm or shorter and which has a low critical magnetic field for evoking a large Barkhausen discontinuity has been desired. Also, the development of a thin-type amorphous alloy material for forming magnetic markers without hardly any twisting has been desired.
Thus, it is an object of the present invention to provide an amorphous alloy ribbon having a length of 10 cm or shorter which exhibits a large Barkhausen discontinuity in a critical magnetic field of 0.7 Oe or lower.
Also, another object of the present invention is to provide a thin-type small-sized magnetic marker comprising the above-described amorphous alloy ribbon which exhibits a large Barkhausen discontinuity.
As a result of various investigations for attaining the above objectives, the present inventors discovered that an Fe group-based amorphous alloy ribbon having a specific cross-sectional form can have magnetic characteristics exhibiting a large Barkhausen discontinuity in a magnetic hysteresis loop even when the length thereof is 10 cm or shorter. Also, only a low critical magnetic field is needed to evoke a large Barkhausen discontinuity, and the characteristics described above can be achieved even in the case of an amorphous alloy ribbon having less twist. The present invention was achieved based on these findings.
That is, in a first embodiment, the present invention provides an Fe group-based amorphous alloy ribbon having a cross section having a width of from 100 to 900 xcexcm and a thickness of from 8 to 50 xcexcm, and having a magnetic hysteresis loop which exhibits a large Barkhausen discontinuity.
In a second embodiment, the present invention provides an Fe group-based amorphous alloy ribbon having a cross-sectional area of from 0.0025 to 0.03 mm2 and having a magnetic hysteresis loop which exhibits a large Barkhausen discontinuity.
In a third embodiment, the present invention provides an Fe group-based amorphous alloy ribbon of the above-described first or second embodiment having a thickness/width ratio of from 0.015 to 0.4.
In a fourth embodiment, the present invention provides an Fe group-based amorphous alloy ribbon prepared by heat-treating a twisted ribbon having a twisting number, when no stress is applied thereto, of from 0.05 to 3.5 turns per 10 cm length of the ribbon, and wherein said amorphous ribbon when held flat has a magnetic hysteresis loop which exhibits a large Barkhausen discontinuity.
Also, in a fifth embodiment, the present invention provides a magnetic marker comprising the Fe group-based amorphous alloy ribbon of the present invention as described above.
The amorphous alloy ribbon of the present invention exhibits a large Barkhausen discontinuity in a critical magnetic field of 0.7 Oe or lower even when the length of the ribbon is 10 cm or shorter. When the amorphous alloy ribbon is placed in an alternating magnetic field, excellent pulse voltage characteristics are obtained in a detection coil. Also, because the twisting number of the amorphous alloy ribbon is reduced, the ribbon is easily handled. As a result, practically usable magnetic markers which scarcely show twisting can be prepared in which the ribbon is held flat with a film of an organic material, etc.
Furthermore, the amorphous alloy ribbon of the present invention can be widely applied to various magnetic sensors such as a rotation sensor, etc. Also, the inventive amorphous alloy ribbon is an industrial material which can be applied to various sensor elements such as a super thin-type pulse generating element, which elements cannot be realized by conventional amorphous alloy filaments.